1. Field of the Invention
The present invention relates to a hollow tubular body having a platinum or platinum alloy hollow tube, suitable for a conduit for molten glass of a glass manufacturing apparatus, wherein an electrode for conduction heating is joined to the outer circumference of the hollow tube.
The present invention relate to a method for heating the hollow tubular body of the present invention by conduction heating.
The present invention relate to a glass manufacturing apparatus employing the hollow tubular body as a conduit for molten glass and a method for producing glass.
2. Discussion of Background
In a glass manufacturing apparatus, a hollow tube made of platinum or a platinum alloy such as a platinum-gold alloy or a platinum-rhodium alloy is employed for a conduit in which molten glass of high-temperature is passed.
In a molten glass feeding apparatus described in JP-A-6-227822 for example, a platinum or platinum alloy pipe is connected to a molten glass outlet located in a lower portion of the glass melting tank. As other examples of the conduit for feeding molten glass, there are a discharge pipe provided to remove impurities from a glass manufacturing apparatus, a discharge pipe for feeding molten glass from a glass manufacturing apparatus to a molding die in order to form an optical element such as lens, prism or the like.
In the glass manufacturing apparatus, the conduit in which molten glass is passed is heated in order to adjust the temperature of the molten glass. There is a case that the conduit is heated from the outside by means of a heat source such as a heater or the like. However, in the case of a platinum or platinum alloy hollow tube, it has been popularly practiced to provide an electrode on the hollow tube to carry out conduction heating.
FIG. 4 is a perspective view showing a conventional platinum or platinum alloy hollow tube provided with an electrode for conduction heating. In FIG. 4, a ring electrode 200 is joined to the outer circumference of a cylindrical hollow tube 100. Two lead-out electrodes 300, 301 are joined to outer edges of the ring electrode 200 so as to oppose each other.
The hollow tube 100, the ring electrode 200 and the lead-out electrodes 300, 301 are made of platinum or a platinum alloy. The hollow tube 100 shown in FIG. 4 is connected to an external power source (not shown) via the lead-out electrodes 300, 301 and is heated by feeding a current from the external power source.
The inventors of this patent application have found that when the hollow tube 100 as shown in FIG. 4 is heated by conduction heating, the current is concentrated to a specified portion of the electrode 200 whereby this portion is subjected to local-overheating. This will be explained in more detail.
In FIG. 4, when the lead-out electrodes 300, 301 are connected to the external power source to carry out conduction heating, a current flows from the lead-out electrodes 300, 301 through the electrode 200 to the hollow tube 100. The ring electrode 200 is with intent of feeding the current uniformly in the whole portion of the hollow tube 100. However, the current from the lead-out electrodes 300, 301 is concentrated to specified portions of the electrode 200.
The current flows through the shortest path according to its characteristic of conduction. In the case of the hollow tube 100 shown in FIG. 4, the current from the lead-out electrodes 300, 301 passes through the shortest path indicated by arrow marks, with the result that the current flowing in the electrode 200 converges at specified portions (portions indicated by dotted lines) of the electrode 200. The portions indicated by the dotted lines of the electrode 200 are subjected to local-overheating due to convergence of the current. The occurrence of such local-overheating may destroy the electrode 200 due to a thermal stress. If the electrode 200 is destroyed, conduction heating to hollow tube 100 becomes impossible whereby there causes a temperature difference between the molten glass and the hollow tube and it is difficult to produce glass.
Further, when the current converges at a specified portion in the electrode 200, the current flowing from the electrode 200 to the hollow tube 100 converges also at a specified portion in the hollow tube 101. Specifically, the current converges at the joint portion indicated by the dotted lines, to the electrode 200 in the circular joint portion to the electrode 200. When the current converges at the specified portion of the hollow tube 100, that portion is subject to local-overheating whereby the hollow tube may be damaged due to a thermal stress or the quality of molten glass passing through the hollow tube 100 may change.